Showing papers on "Crystallization published in 1998"

Crystalline Polymorphism of Organic Compounds

Abstract: Crystal polymorphism is encountered in all areas of research involving solid substances. Its occurrence introduces complications during manufacturing processes and adds another dimension to the complexity of designing materials with specific properties. Research on polymorphism is fraught with unique difficulties due to the subtlety of polymorphic transformations and the inadvertent formation of pseudopolymorphs. In this report, a summary of thermodynamic, kinetic and structural considerations of polymorphism is presented. A wide variety of techniques appropriate to the study of organic crystalline polymorphism and pseu-dopolymorphism is then surveyed, ranging from simple crystal density measurement to observation of polymorphic transformations using variable-temperature synchrotron X-ray diffraction methods. Application of newer methodology described in this report is yielding fresh insights into the nature of the crystallization process, holding promise for a deeper understanding of the phenomenon of polymorphism and its practical control.

Crystallization and Melting Behavior of Polylactides

Abstract: Six polylactides, polymerized with Salen−Al−OCH3 initiator and having optical purities between 43% and 100%, were analyzed by differential scanning calorimetry, X-ray diffraction, and optical microscopy, following various crystallization conditions. It was found that each of those polylactides can crystallize, even those with low optical purities; their crystallization rate is, however, slower than those for high optical purity polyesters. Moreover, the low optical purity polymers tend to form stereocomplexes between the l and d sequences of the same polylactide, which behavior is ascribed to their multiblock microstructure. A correlation was found between the measured melting temperature of optically active polylactides and their average sequence length.

Crystallization of Nucleic Acids and Proteins: A practical Approach

Abstract: Giege and Ducruix: An introduction to the crystallogenesis of biological macromolecules Lorber and Giege: Biochemical aspects and handling of macromolecular solutions and crystals Berne, Doublie and Carter, Jr: Molecular biology for structural biology Carter, Jr: Experimental design, quantitive analysis, and the cartography of crystal growth Ducruix and Giege: Methods of crystallization Robert et al: Crystallization in gels and related methods Stura: Seeding techniques Dock-Bregeon, Moras and Giege: Nucleic acids and their complexes Reiss-Husson and Picot: Crystallization of membrane proteins Ries-Kaut and Ducruix: From solution to crystals with a physico-chemical aspect Veesler and Boistelle: Diagnostic of prenucleation and nucleation by spectroscopic methods and background on the physics Brisson, Lambert and Bergsma-Schutter: Two-dimensional crystallization of soluble proteins on planar lipid films Stura and Gleichmann: Soaking techniques Sawyer and Turner: X-ray analysis.

Measurement of the self-intermediate scattering function of suspensions of hard spherical particles near the glass transition

Abstract: Dynamic light-scattering measurements are reported for suspensions at concentrations in the vicinity of the glass transition. In a mixture of identically sized but optically different particles having hard-sphere-like interactions, we project out the incoherent (or self-) intermediate scattering functions by adjusting the refractive index of the suspending liquid until scattering from the structure is suppressed. Due to polydispersity, crystallization is sufficiently slow so that good estimates of ensemble-averaged quantities can be measured for the metastable fluid states. Crystallization of the suspensions is still exploited, however, to set the volume fraction scale in terms of effective hard spheres and to eliminate (coherent) scattering from the structure. The glass-transition volume fraction is identified by the value where large-scale particle motion ceases. The nonequilibrium nature of the glass state is evidenced by the dependence on the waiting time of the long time decay of the relaxation functions. The self-intermediate scattering functions show negligible deviation from Gaussian behavior up to the onset of large-scale diffusion in the fluid or the onset of waiting time effects in the glass.

Crystal grain nucleation in amorphous silicon

Abstract: The solid phase crystallization of chemical vapor deposited amorphous silicon films onto oxidized silicon wafers, induced either by thermal annealing or by ion beam irradiation at high substrate temperatures, has been extensively developed and it is reviewed here. We report and discuss a large variety of processing conditions. The structural and thermodynamical properties of the starting phase are emphasized. The morphological evolution of the amorphous towards the polycrystalline phase is investigated by transmission electron microscopy and it is interpreted in terms of a physical model containing few free parameters related to the thermodynamical properties of amorphous silicon and to the kinetical mechanisms of crystal grain growth. A direct extension of this model explains also the data concerning the ion-assisted crystal grain nucleation.

Crystallization Behavior and Phase Formation in Zr–Al–Cu–Ni Metallic Glass Containing Oxygen

Abstract: Differential scanning calorimetry, X-ray diffraction and transmission electron microscopy were used to study the thermal stability and the phase formation during annealing of (Zr 0.65 Al 0.075 Cu 0.175 Ni 0.10 ) 100-x O x metallic glasses (0.2≤x≤ 0.8) prepared by melt spinning. Increasing oxygen content changes the crystallization mode from a single to a double step process. The glass transition temperature T g increases slightly with increasing oxygen content whereas the crystallization temperature T x decreases, causing a reduction of the undercooled liquid region. The deterioration of the thermal stability is related to oxygen-induced formation of metastable quasicrystalline and fcc NiZr 2 -type phases which coexist with tetragonal Cu(Al, Ni)Zr 2 and hexagonal Zr 6 NiAl 2 for x=0.2 and 0.4. For x=0.8 only the quasicrystalline phase is observed in the first step of crystallization. The details of phase formation and transformation during crystallization depend strongly on the annealing conditions. At elevated temperatures the quasicrystals transform into Cu(Al, Ni) Zr 2 and the fcc phase transforms into the stable Zr 6 NiAl 2 compound. Increasing oxygen content leads to more pronounced metastable phase formation in the first step of crystallization at lower temperatures, and extends the stability regime of these phases to higher temperatures. For annealing temperatures ≥ 873 K the samples exhibit the same crystallization products regardless of the oxygen content of the material.

Aluminum-induced crystallization of amorphous silicon on glass substrates above and below the eutectic temperature

Abstract: The achievement of high-quality continuous polycrystalline silicon (poly-Si) layers onto glass substrates by using aluminum-induced crystallization is reported. The crystallization behavior of dc sputtered amorphous silicon on glass induced by an Al interface layer has been investigated above and below the eutectic temperature of 577 °C. Secondary electron micrographs in combination with energy-dispersive x-ray microanalysis show that annealing below this temperature leads to the juxtaposed Al and Si layers exchanging places. The newly formed poly-Si layer is fully crystallized and of good crystalline quality, according to Raman spectroscopy and transmission electron microscopy investigations. At 500 °C, the time needed to crystallize a 500-nm-thick Si layer is as short as 30 min. By annealing above the eutectic temperatures, layer exchange is not as pronounced and the newly formed Al layer is found to contain a network of crystallized Si.

Influence of spreading rate and magma supply on crystallization and assimilation beneath mid‐ocean ridges: Evidence from chlorine and major element chemistry of mid‐ocean ridge basalts

Abstract: Chlorine and major elements in >400 mid-ocean ridge basalt (MORB) glasses from 20 suites are used to examine how spreading rate, magma flux, tectonics, and hydrothermal activity influences assimilation and crystallization beneath MOR. Crystallization depths were determined for fractionated glasses using published models that describe liquids saturated with olivine+ clinopyroxene+ plagioclase. Calculated depths are minima for the onset of crystallization and maxima for the completion of crystallization for each liquid. Glasses from fast spreading ridges and from medium and slow spreading ridges with low Na8.0 define low-pressure liquid lines of descent (LLDs). Higher crystallization pressures and greater variability are obtained from slow and medium spreading ridges with high Na8.0. Crystallization pressures do not vary regularly along individual segments. The correlation between average crystallization pressure and Na8.0 suggests that magma supply (and perhaps mantle temperature) plays an important role in determining magma ascent and crystallization depths. Cl/K in glasses is an indicator of assimilation of hydrothermally influenced material. Suites of MORB with high crystallization pressures have low Cl/K: from below detection limits (≈0.01) in normal MORB (NMORB) to about 0.05–0.08 in enriched MORB (EMORB). We propose that this trend defines the mantle limit of Cl/K and that higher values are related to assimilation. Cl/K is highest (up to 1.1) and is negatively correlated with MgO along the superfast spreading southern East Pacific Rise (EPR) and the propagating, low-Na8.0 Galapagos Spreading Center (GSC) at 85°W. Cl/K is also above mantle values, but is not well correlated with MgO, in MORB from fast and medium spreading ridges and from slow spreading ridges that have low Na8.0 and low crystallization pressures, e.g., Reykjanes Ridge. Cl/K is not correlated with crystallization pressure for individual samples within any suite. We propose that the spreading rate and the extent of melting act together to determine the total magma flux to a ridge, which influences crustal temperatures and determines how magmas ascend. At the highest magma fluxes, Cl/K is correlated with MgO, consistent with continuous assimilation of material that has a uniform Cl content: crystallization and assimilation are steady state processes that occur in crustal magma bodies that are larger than the scale of crustal heterogeneity in Cl. The lowest magma fluxes occur on slow spreading ridges that have formed by small extents of melting. In this cooler environment, magma crystallizes at the base of the strong lithosphere, below the level of alteration, and then ascends rapidly with little crystallization at shallow levels, so Cl contamination is avoided. On slow and medium spreading ridges with high extents of melting, magma flux is intermediate and forms small or transient crustal magma bodies: whether a magma batch becomes enriched in Cl depends upon the particular crust that it encounters.

Electronic Crystallization in a Lithium Battery Material: Columnar Ordering of Electrons and Holes in the Spinel LiMn 2 O 4

Abstract: ${\mathrm{LiMn}}_{2}{\mathrm{O}}_{4}$ presents a first order structural transition at 290 K that was known to perturb the functioning as cathode in rechargeable Li batteries. We have solved the structure at 230 K and deciphered unambiguously the nature of this phase transition. The analysis of valence bond sums shows that the transition results from a partial charge ordering: two of the five Mn sites correspond to well-defined ${\mathrm{Mn}}^{4+}$ and the other three sites are close to ${\mathrm{Mn}}^{3+}$ ions. Charge ordering is accompanied by simultaneous orbital ordering due to the Jahn-Teller effect in ${\mathrm{Mn}}^{3+}$ ions. The microscopic details obtained from the structure are crucial for understanding the electron hopping persisting below the transition.

Characterization of magnetically oriented phospholipid micelles for measurement of dipolar couplings in macromolecules.

Abstract: Weak alignment of solute molecules with the magnetic field can be achieved in a dilute liquid crystalline medium, consisting of an aqueous mixture of dimyristoyl-phosphatidylcholine (DMPC) and dihexanoyl-phosphatidylcholine (DHPC). For a certain range of molar ratios, DMPC and DHPC can form large, disc-shaped particles, commonly referred to as bicelles (Sanders and Schwonek, 1992), which cooperatively align in the magnetic field and induce a small degree of alignment on asymmetrically shaped solute molecules. As a result, dipolar couplings between pairs of 1H, 13C or 15N nuclei are no longer averaged to zero by rotational diffusion and they can be readily measured, providing valuable structural information. The stability of these liquid crystals and the degree of alignment of the solute molecules depend strongly on experimental variables such as the DMPC:DHPC ratio and concentration, the preparation protocol of the DMPC/DHPC mixtures, as well as salt, temperature, and pH. The lower temperature limit for which the liquid crystalline phase is stable can be reduced to 20 degrees C by using a ternary mixture of DHPC, DMPC, and 1-myristoyl-2-myristoleoyl-sn-glycero-3-phosphocholine, or a binary mixture of DHPC and ditridecanoyl-phosphatidylcholine. These issues are discussed, with an emphasis on the use of the medium for obtaining weak alignment of biological macromolecules.

Genesis and Structure of WOx/ZrO2 Solid Acid Catalysts†

Abstract: Zirconia-supported tungsten oxide catalysts were prepared by suspending hydrous zirconium oxide in aqueous solutions of ammonium metatungstate at their natural pH. The suspensions were refluxed at 383 K followed by evaporation of the water, drying and calcination. The WO3 loadings were chosen between 3.6 and 32 wt % and the calcination temperatures were in the range 773 ≤ T ≤ 1273 K. The resulting materials were structurally characterized by X-ray diffraction, differential thermal analysis, and surface area measurements, by laser Raman and Fourier transform infrared (FTIR) spectroscopy, and by UV−vis diffuse reflectance spectroscopy. Their acidic properties were tested by FTIR spectroscopy using carbon monoxide as a probe molecule. The experimental results indicate that the presence of WOx retards the crystallization of the zirconia material and stabilizes the tetragonal ZrO2 phase and the intrinsic BET surface area. W−O−W linkages are detected even at the lowest WO3 loadings indicating that oligomeric WO...

X‐Ray Photoelectron Spectroscopy and Scanning Tunneling Microscopy Study of Passive Films Formed on (100) Fe‐18Cr‐13Ni Single‐Crystal Surfaces

Abstract: X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) were combined to investigate the thickness, chemical composition, and structure of passive films formed in 0.5 M H 2 SO 4 on (100)Fe-18Cr-13Ni. The XPS measurements show that aging under polarization at +500 mV/SHE causes a dehydration reaction of the outer chromium hydroxide layer of the passive film. This reaction results in a thickening of the mixed Cr(III) and Fe(III) inner oxide layer and increases the Cr 2 O 3 enrichment. This reaction consumes, in addition to chromium hydroxide of the outer layer, chromium from the metallic phase underneath the passive film. Only traces of nickel (hydroxide) are detected in the passive film, whereas Ni enrichment is observed in the alloy underneath the passive film. High-resolution STM images reveal that aging under polarization causes a crystallization of the inner Cr 2 O 3 oxide layer in epitaxy with the substrate. The epitaxial relationship is (0001) α-Cr 2 O 3 ||(100) Fe-18Cr-13Ni with [2130]α-Cr 2 O 3 ||[011] Fe-18Cr-13Ni. The crystallization proceeds with a faster kinetics than on (110) Fe-22Cr in the same conditions. The crystallization rate is modified by the presence of Ni in the alloy, which is enriched in the metallic phase underneath the film and slows down the formation of Cr 2 O 3 in the inner part of the film. This favors a more complete process of crystallization. Aging under polarization is beneficial to the further stability of the passive film in air.

Thermal and Mechanical Properties of Ti–Ni–Cu–Sn Amorphous Alloys with a Wide Supercooled Liquid Region before Crystallization

Abstract: A wide supercooled liquid region exceeding 50K before crystallization was observed for melt-spun Ti 50 Ni 25-x Cu 25 Sn x (x=3 and 5 at%) amorphous alloys. The temperature interval of the supercooled liquid region defined by the difference between crystallization temperature (T x ) and glass transition temperature (T g ), Δ T x (= T x -T g ) is 40 K for the 0%Sn alloy and increases to 50 K for the 3%Sn alloy and 60 K for the 5%Sn alloy. With increasing Sn content, the ΔT x value decreases significantly. A similar increase in thermal stability of the supercooled liquid was also recognized for the 3 at%Sb-containing alloy in the Ti 50 Ni 25-x Cu 25 Sb x system. The replacement by 3 to 5 at%Sn for Ni also induces an increase of mechanical strength, in addition to the increase in Δ T x . The tensile fracture strength (σ f ), Young's modulus (E) and Vickers hardness (H v ) increase from 1800 MPa, 93 GPa and 530, respectively, for the Ti 50 Ni 25 Cu 25 alloy to 2050 MPa, 102 GPa and 650, respectively, for the Ti 50 Ni 20 Cu 25 Sn 5 alloy. The crystallization takes place through a single exothermic reaction, accompanying the simultaneous precipitation of multiple (CuTi + Cu 4 Ti 3 + NiTi) phases. The crystallization mode requires long-range atomic rearrangements for precipitation of the crystalline phases, leading to the increase in the stability of supercooled liquid against crystallization. The high thermal stability of the supercooled liquid enabled the production of bulk amorphous alloys in the diameter range up to about 6 mm by copper mold casting. There is no appreciable difference in the stability of the supercooled liquid region between the melt-spun and cast bulk amorphous alloys. The first synthesis of the Ti-based amorphous alloys with high glass-forming ability and good mechanical properties allows us to expect the future development of bulk amorphous alloys as a new type of high specific strength material.

Nickel induced crystallization of amorphous silicon thin films

Abstract: Nickel (Ni) induced crystallization of amorphous silicon (a-Si) has been studied by selective deposition of Ni on a-Si thin films. The a-Si under and near the Ni-covered regions was found to be crystallized after heat treatment at 500 °C from 1 to 90 h. Micro-Auger electron spectroscopy revealed that a large amount of Ni stayed in the region under the original Ni coverage, but no Ni was detected either in the crystallized region next to the Ni coverage or in the amorphous region beyond the front of the laterally crystallized Si. X-ray photoelectron spectroscopy revealed a nonuniform Ni distribution through the depth of the crystallized film under the original Ni coverage. In particular, a Ni concentration peak was found to exist at the interface of the crystallized Si and the buried oxide. It was found that a layer of 5-nm-thick Ni could effectively induce lateral crystallization of over 100 μm of a-Si, but the lateral crystallization rate was found to decrease upon extended heat treatment. Transmission e...

Epitaxial Crystallization and AFM Investigation of a Frustrated Polymer Structure: Isotactic Poly(propylene), β Phase

Abstract: The metastable phase of isotactic polypropylene (βiPP) is crystallized epitaxially on two specific nucleating agents: γ-quinacridone and dicyclohexylterephthalamide (DCHT). The resulting thin film...

Crystallization of Adsorbed Polymer Monolayers

Abstract: We present atomic force microscopy results and corresponding computer simulations on crystallization of quasi-2D monolayers of polyetheleneoxid adsorbed onto bare silicon wafers. Fingerlike branched patterns with a characteristic width $w$ resulted from crystallization at temperatures ${T}_{c}$ below the melting point ${T}_{m}$. $w$ decreased exponentially as $({T}_{m}{\ensuremath{-}T}_{c})$ increased. The patterns are explained by a simple model considering the interplay of transport on the surface and the probability of attachment to the crystal.

Phase transitions and polymorphism of cocoa butter

Abstract: The polymorphism and phase transitions of cocoa butter (CB) have been reexamined separately by differential scanning calorimetry (DSC) and X-ray diffraction as a function of temperature (XRDT) at scanning rates between 0.1 to 5°C/min and 0.1 to 2°C/min, respectively. A new instrument, which allowed simultaneous DSC and XRDT recordings from the same sample by taking advantage of the high-energy flux of a synchrotron, was employed for characterization of the intermediate phase transitions. These techniques allowed us to confirm the existence of the six polymorphic forms of CB (called I to VI) by in situ characterization of their formation in the DSC + XRDT sample holder. A detailed study of Form I structure led us to propose a liquid-crystal organization in which some of the chains displayed sharp long-spacing lines (d001=52.6±0.5 A) and a β′ organization (4.19 and 3.77 A), while the others remained unordered with broad scattering (maxima at about 112 and 36.5 A). The organization of this liquid crystalline phase is compared to that of fat and oil liquids. This liquid crystalline phase progressively transformed on heating into a more stable phase (Form II, α type, d001=48.5±0.5 A and short-spacing at 4.22 A). Form III was only observed in a sharp temperature domain through its specific short-spacings. The existence of the six species has been essentially related to the crystallization of monounsaturated triacylglycerols (TAG), while trisaturated species were found partly solid-soluble in these six polymorphic forms. An insoluble fraction crystallized independently of the polymorphism of the monounsaturated TAG in a separate phase with long-spacings that were either of the α (49.6±0.5 A) or β (44.2±0.5 A) form. In mixture with Form V, this fraction melts and solubilizes in the liquid phase at 37.5°C. Isolation of these high-melting crystals shows a melting point of about 50°C. High-performance liquid chromatography analysis of this fraction confirmed an increase from 3.0 to 11.3% of saturated TAG and their association with part of the 1,3-stearoyl-2-oleoylglycerol preferentially to 1-palmitoyl-2-oleoyl-3-stearoylglycerol and (1,3-palmitoyl-2-oleoylglycerol).

Ferromagnetic Bulk Amorphous Alloys

Abstract: This article reviews our recent results on the development of ferromagnetic bulk amorphous alloys prepared by casting processes. The multicomponent Fe-(Al,Ga)-(P,C,B,Si) alloys are amorphized in the bulk form with diameters up to 2 mm, and the temperature interval of the supercooled liquid region before crystallization is in the range of 50 to 67 K. These bulk amorphous alloys exhibit good soft magnetic properties, i.e., high B s of 1.1 to 1.2 T, low H o of 2 to 6 A/m, and high μ e of about 7000 at 1 kHz. The Nd-Fe-Al and Pr-Fe-Al bulk amorphous alloys are also produced in the diameter range of up to 12 mm by the copper mold casting process and exhibit rather good hard magnetic properties, i.e., B r of about 0.1 T, high H o of 300 to 400 kA/m, and rather high (JH)max of 13 to 20 kJ/m3. The crystallization causes the disappearance of the hard magnetic properties. Furthermore, the melt-spun Nd-Fe-Al and Pr-Fe-Al alloy ribbons exhibit soft-type magnetic properties. Consequently, the hard magnetic properties are concluded to be obtained only for the bulk amorphous alloys. The bulk Nd- and Pr-Fe-Al amorphous alloys have an extremely high T x/Tm of about 0.90 and a small ΔT m(=T m−T x) of less than 100 K and, hence, their large glass-forming ability is due to the steep increase in viscosity in the supercooled liquid state. The high T x/Tm enables the development of a fully relaxed, clustered amorphous structure including Nd-Nd and Nd-Fe atomic pairs. It is, therefore, presumed that the hard magnetic properties are due to the development of Nd-Nd and Nd-Fe atomic pairs with large random magnetic anisotropy. The Nd- and Pr-based bulk amorphous alloys can be regarded as a new type of clustered amorphous material, and the control of the clustered amorphous structure is expected to enable the appearance of novel functional properties which cannot be obtained for an ordinary amorphous structure.